In-situ measurements of species in process streams are preferred over extractive methods of analysis because of the uncertainty that extracted samples remain representative of the process streams once they are removed and because of the extended time required for laboratory analyses. In-situ optical methods of analysis require the installation of windows on a process stream pipe, duct, or other housing, through which a light can enter and/or exit the process environment. This enables spectroscopic determinations of atomic or molecular species concentrations or measurements of physical characteristics of process streams such as temperatures, velocities, or particulates.
An inherent problem with the use of optical windows in making such measurements is the tendency of solid particles to become deposited on the interior surface of the windows. Deposits on the windows may arise from sources such as dust, process particles, or condensible vapors carried in the process stream. A layer of particles builds up on optical windows, attenuating light beams and interfering with optical measurements. A solution to this problem is especially needed for prevention of window contamination in apparatus used for study of coal conversion processes, where high temperatures, pressures, and contamination levels are frequently encountered, and where a need exists for unobstructed access for extended time intervals. For such applications, the design used to minimize window contamination must avoid producing chemical and physical changes to the process stream in the region being monitored.
Various approaches have been taken to remove or compensate for particle deposition on optical windows of process stream equipment. The attenuation of light beams by particulate deposits has been compensated for to some degree by using as a reference a parallel light source which is not absorbed by the process stream constituents. Both the signal and reference beam will be attenuated to the same degree by the layer of particles on the window. Thus, the degree of attenuation due to particles on the window is known, and the signal beam intensity measurements can be adjusted accordingly. This technique is useful for thin layers of deposits; however, as deposition on the windows continues, optical measurements become impossible, and the windows have to be cleaned. Cleaning the windows usually requires shutting down the process. Thus, frequent cleaning of optical windows is inconvenient and, in many cases, impossible.
Clean gas jets impinging on the interior surface of windows have been used to reduce the rate of particle deposition. However, this technique is often ineffective. This technique has the further disadvantage that the gas jet used to purge the windows mixes with the process stream, changing its composition and temperature profiles. Relatively large volume flows of purge gas are required to substantially reduce the rate of particle deposition.